1. Field of the Invention
The present invention generally relates to switching between two power sources and more particulary to a system and method of switching power between an internal battery and an external DC adapter without loss or interruption of power.
2. Status of the Prior Art
Electronic devices such as laptop computers, cell phones, PDA's, etc . . . , can be powered using either an internal battery power source or an external power source. For example, a cell phone may use a Li-Ion rechargeable battery as the internal power source and a DC adapter as the external power source. When the external power source is plugged into the device, it disables power from the internal power source so that the external power source is the sole provider of power. Similarly, when the external power source is removed from the device, power is switched to the internal power source.
The switching between the internal power source and the external power source can be accomplished through the use of a FET switch. Referring to FIG. 1, a prior art switching circuit 10 using a MOSFET switch U4 is shown. The MOSFET switch U4 has switching transistors 12 and 14, as well as diodes 16 and 18. The prior art switching circuit further includes resistor R 102 and diode D10, as shown in FIG. 1.
When the DC adapter is plugged into the device, the voltage VADAPTER increases gradually and the MOSFET switch U4 is turned off thereby switching off the power from VBAT. However, there is a delay in supplying the power from the DC adapter such that the voltage at VOUT will drop to zero. A switching voltage regulator that draws current all the time may be connected to VOUT. The delay in supplying power from the DC adapter creates the situation whereby there may be no current going to the switching regulator thereby resulting in 0 volts at VOUT for a short period of time. This voltage drop on the switching regulator is transferred to the output of the regulator such that brownout conditions may be created on sensitive digital integrated circuits.
Similarly, when the DC adapter is unplugged from the device, the MOSFET switch U4 will switch power back to the battery. Yet, when the device is unplugged, the voltage on the gates of the switching transistors 12 and 14 will decrease slowly. During this period of discharge, the MOSFET switch U4 is not fully on and incapable of supplying enough current from the battery. As such, there is again a drop in voltage at VOUT.
The present invention addresses the above-described deficiencies in switching between power supplies by providing a circuit which instantaneously switches between power sources. The switching circuit of the present invention avoids extensive voltage drop at the output of the switching circuit during DC adapter turn-on and turn-off such that brownouts are avoided.